A gray level image comprises a multiplicity of pixels, each pixel represented by a digital value indicative of a gray level or a color value. Each gray level corresponds to a color tone between 0% and 100%. When such a gray level image is passed to a black/white binary laser printer, the gray level values are converted to binary pixel values. The laser printer then attempts to reproduce the gray levels of the image through use of dither procedures or other approximation methods.
The prior art binary laser printer has been succeeded by gray level laser printers which modulate a laser beam's intensity or duration to achieve a variation in charge state at each pixel location on a photoconductor which, when subsequently toned, produces a highly faithful gray level reproduction.
Edge definition is a problem in binary pixel images due to the stepped pixel arrangement employed to represent boundaries of characters and images. The prior art has employed a procedure, generally called "Resolution Enhancement technology" (REt), to smooth edges by modulating pixels in accordance with comparisons made to pre-stored template arrangements. U.S. Pat. Nos. 5,005,139 and 4,847,641, both to C. C. Tung and assigned to the same Assignee as this application, describe a hardware-based method for accomplishing such REt edge enhancement. The disclosure of the Tung patents is incorporated herein by reference. Briefly stated, the Tung procedure logically tiles "windows" over successive bit patterns of an image. Each image pixel window created by the tiling procedure comprises a center pixel and surrounding neighbor pixels. Each image pixel window is thus a pattern of pixels from the original binary image.
Each image pixel window is compared in parallel, in the Tung system, against hundreds of templates, each template defining an expected edge pixel arrangement which can be improved by modification of the central pixel in the image pixel window. Each template is associated with a correction pixel for the central pixel of the image pixel window.
The central pixel and neighbor pixels within the image pixel window are compared against the templates, in parallel, via a gate array so as to determine in one or very few clock cycles which template, if any, matches the pixel arrangement within the image pixel window. If a template is found to match the image pixel window pattern, the associated correction pixel value is output and is substituted for the central pixel in the image pixel window.
Thus, as the image pixel window is stepped across the image, corrections are made to any pixel arrangement therein which matches a template so as to improve edge presentation of the binary image. REt has proved successful in improving print quality of binary pixel value images.
Presently, two types of fonts are widely used in laser printers, i.e., bitmapped and scalable. A scalable font includes characters that are defined as a series of contours (i.e., functions) so that when provided with a desired point size, produce a precise character of the desired shape and point size. Bitmapped fonts differ from a scalable font in that they cannot be changed in point size.
Laser printers that utilize gray level pixel values may employ four, six or eight bits to define each pixel's gray value. It is desirable to improve the smoothness of a gray level scalable font by adjusting the gray levels of edge pixels of each font character. The prior art has, as a result, employed software, firmware and hardware-implemented edge smoothing in gray level laser printers.
Frasier et ah, in U.S. Pat. Nos. 5,134,495 and 5,193,008, employ an edge smoothing technique which changes laser pulse exposure times in accordance with template comparisons, to selected pixels in an image. Their process initially creates a binary image at a level of resolution (e.g., 600 dots per inch (dpi)) that is higher (e.g., double) than that which a printer can output. A logical window is then stepped, at 600 dpi, across the entire image plane. At each step of the window, the higher resolution pixel arrangement is compared to pre-existing templates and, upon a match, causes an altered modulation of the printer's laser beam. The modulated laser beam produces on a photoreceptor not only a gray level central pixel at the lower resolution, but also sufficiently exposes the photoreceptor about the edges of a central pixel to enable scans by adjacent scan lines to combine to create intermediate pixels between the scan lines which provides an edge smoothing effect.
More specifically, pixel dots are created half-way between adjacent horizontal scan lines by energizing a plurality of pixels on adjacent scan lines so that the sum of the energies applied to intermediate pixel points is above a threshold level--thereby enabling the intermediate pixel points to be later toned. Frasier et ah employ a "look up table" based upon a template view of the source bitmap.
In the '008 patent, Frasier et al. operate in a similar manner to that described in the '495 patent, but utilize a 1.times.3 pixel window which enables a much simplified logic to be utilized to control the decision making process regarding pulse width modulation of the printer's laser.
Both of the Frasier et al. patents teach that the entire image plane is created at a higher level of resolution, thereby requiring a substantial memory allocation. Further, to carry out the edge-smoothing procedure taught in the '495 patent, many templates must be provided (in the form of a look-up table) to enable window comparisons to be made and pulse width modulations to be derived. The edge-smoothing procedure described in the '008 patent does not enable the obtaining of all possible combinations of interleaved pixels, but requires less logic to obtain intermediate pixel representations and a resultant edge-smoothing action.
It is an object of this invention to provide an improved software-based edge-smoothing procedure for a gray level laser printer.
It is another object of this invention to provide an improved edge-smoothing procedure wherein required memory capacity is minimized.